An undesirable result of burning coal in boilers is the deposition of ash on heat exchanger surfaces, for example, in the superheater and reheater regions of steam generating units. Ash deposition adversely affects the reliability, availability and predictability of performance of coal-fired boiler units, in particular those which burn so-called "lower rank" coals. The ash component of coal consists of inorganic compounds of various compositions and properties. Whatever the composition or properties of a particular ash component may be, however, it is known that ash deposition rate can be reduced by (among other things) increasing the fineness of pulverized coal ("Technology and Use of Lignite"--Proceedings: Bureau of Mines--University of North Dakota Symposium, Grand Forks, N. Dakota, May 1-2 1969 --Bureau of Mines Information Circular 8471, pages 109-111). Fine particles in size ranges such that their motions in a gas stream are significantly governed by Stokes' Law, tend to follow the gas stream through the boiler and heat exchangers to and up the flue, where they can be captured in electrostatic precipitators or otherwise treated after leaving the boiler. In a gaseous medium, such as air, the motions of the very small particles of both coal and ash, many of which have essentially the same effective aerodynamic diameters, are governed essentially by Stokes' Law defining resistance to motion, EQU R=6.pi.nav
where "n" is the fluid viscosity, "a" is the radius of the particle (sphere), and "v" is the velocity of the particle.
Coarser particles tend to continue in straight-line paths, and do not readily follow the gas stream, and so they are more likely to impinge and deposit upon heat exchanger surfaces. The referenced publication illustrates, in FIG. 51 at page 109, that ash fineness has a tremendously important effect on particle impingment; increasing the fineness from 60 to 80 percent through a 200-mesh screen reduced the deposition by approximately one-half in a 30-minute exposure time.
If an electric power utility using coal-fired steam boilers is designed for 80% boiler availability and, due to being able to obtain only a poor quality coal having a higher-than-desired ash content, the boiler availability is reduced to 70%, the loss to the utility is one-eighth of its design rated power. Given a sales price of 5.cent. per Kwh for its product, and a generating plant that is design-rated at 1000 megawatts for 80% boiler availability, the loss of revenue due to one-eighth decreased boiler availability is $150,000 per day!
One way to reduce the size of ash particles to a range in which boiler availability would be significantly increased would accordingly appear to be to pulverize the feed coal so fine that the largest size particle would become 74 microns (200 mesh), or perhaps 37 microns (400 mesh), but this procedure would introduce problems of cost and of handling particles as small as 2 or 3 microns which would be included in the feed coal. However, such a pulverization would result in a flame that would be substantially shorter and hotter than boilers are commonly designed for, resulting in inferior performance and possible damage to portions of the boiler. In addition, finer pulverization would require substantially greater energy. According to the present invention, it is not necessary to pulverize the feed coal so that all of it will pass a 200-mesh sieve.